A wide variety of detection systems have been employed to sense conditions of sheet feed along a use path, e.g. the feed path of original or copy sheets in copier apparatus, the feed path of documents to be scanned by input scanner devices or the feed path of print sheets within printing machines. Such systems have been used to detect (i) sheet position error (e.g. longitudinal, lateral or skew error), (ii) double sheet feed and (iii) improper sheet type (e.g. transparency versus opaque sheet).
U.S. Pat. Nos. 3,230,519; 3,591,922; 3,646,372 and 4,258,326 disclose the use of capacitive sensing devices to detect the presence or absence of dielectric objects, a variation in thickness because of a web splice and the existence of a double sheet feed. Those systems are not used for accurate movement detection.
In many printing, copying and scanning applications, it is important that movement of fed sheets be very accurate. For example, in situations where image portions of different colors are to be placed on print or copy sheets during successive passes, accurate sheet movement positioning is necessary to assure proper register of the different color image portions. When data is to be printed in proper register on a preprinted form or when personalized printing is to be added to advertising literature, accurate movement of the blank form or advertising signature sheet is important. In systems where input scanners merge data from separate input documents, accurate relative location information can require accurate movement of the input and output sheets.
Commonly assigned U.S. Pat. No. 5,035,415, of July 30, 1991, entitled "System for Detecting the Accurate Positioning of Sheets Along a Feed Path", filed in the names of Lee, Kriegel and Stephany describes a system employing a pair (s) of capacitive sensors disposed in a bridge circuit in a manner which allows precise position detection of a fed sheet vis a vis the feed path.